Doing file operations with 64-bit addresses in C + MinGW32

Question

I'm trying to read in a 24 GB XML file in C, but it won't work. I'm printing out the current position using ftell() as I read it in, but once it gets to a big enough number, it goes back to a small number and starts over, never even getting 20% through the file. I assume this is a problem with the range of the variable that's used to store the position (long), which can go up to about 4,000,000,000 according to http://msdn.microsoft.com/en-us/library/s3f49ktz(VS.80).aspx, while my file is 25,000,000,000 bytes in size. A long long should work, but how would I change what my compiler(Cygwin/mingw32) uses or get it to have fopen64?

Answer 1

The ftell() function typically returns an unsigned long, which only goes up to 2³² bytes (4 GB) on 32-bit systems. So you can't get the file offset for a 24 GB file to fit into a 32-bit long.

You may have the ftell64() function available, or the standard fgetpos() function may return a larger offset to you.

Answer 2

You might try using the OS provided file functions CreateFile and ReadFile. According to the File Pointers topic, the position is stored as a 64bit value.